fs/resctrl: Prevent deadlock and use-after-free in info file handlers

resctrl provides files under the info/ directory to expose global
configuration and capabilities to userspace. These files are instantiated
statically during filesystem mount and expose data associated with internal
schema structures via kernfs private pointers.

A potential deadlock exists between userspace readers of these info files
and the unmount filesystem teardown process. Reading an info file invokes
kernfs which acquires an active reference, after which the handler typically
attempts to acquire the rdtgroup_mutex.

Concurrently, unmounting the filesystem holds the rdtgroup_mutex and then
attempts to recursively remove the info kernfs nodes involving kernfs_drain()
which blocks until all active references are released.

Another problem exists where info files might be accessed from an outdated
mount if the filesystem is unmounted and remounted during a reader's
execution, leading to a use-after-free when reading the now-deleted private
schema data.

Introduce info_kn_lock() and info_kn_unlock() helpers to coordinate locking
across all info handlers. These helpers mirror similar logic used by resource
group handlers by deliberately breaking the kernfs active protection before
attempting to acquire the rdtgroup_mutex, preventing the deadlock.

To guard against the vulnerability from rapid mount cycling, info_kn_lock()
securely walks the parent lineage of the kernfs node under an RCU section to
confirm the node belongs to the globally active root before permitting the
operation to proceed. Convert all info file handlers to use this helper and
only de-reference the schema after it is determined safe to do so.

Make no attempt to output an error message to last_cmd_status on failure
since failure implies there is no filesystem with which to display the error
to user space.

  [ bp: Massage commit message. ]

Closes: https://sashiko.dev/#/patchset/20260515193944.15114-1-tony.luck%40intel.com?part=3
Reported-by: Sashiko <sashiko-bot@kernel.org>
Assisted-by: GitHub_Copilot:gemini-3.1-pro
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://patch.msgid.link/0b5238486bd058704d908d39a75aff2815bd18aa.1783963505.git.reinette.chatre@intel.com

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